Silver dollars, the smaller spotted one (
argentus) would do well in a 55 gallon tank with neons and they are from S. America, too.
Argentus doesn't get as tall or as long as the other dollars, so the narrowness of the 55 (13") won't be as much of a problem. It's a fair sized fish, 6-8", and very pretty. Cousins to the pirahna, but naturally herbivore, they will eat some plants and will accept just about anything you want to feed them. Anubias, crinum and green melons are pretty safe from them if the plants are fairly mature, but they may nibble on new leaves. Any other plant is fair game
Dollars can be very timid if they do not have tall vegetation (real or fake) as a refuge or a large enough tank. Once they have ample hiding spots and lots of swimming room, they often shed their shyness and swim out front.
I have six dollars in my 65g, three of which are
argentus, and a school of rasboras and a school of rummy nose tetras.
Roan